DESCRIPTION

SSL_CTX_set_tmp_rsa_callback() sets the callback function for ctx to be
used when a temporary/ephemeral RSA key is required to tmp_rsa_callback.
The callback is inherited by all SSL objects newly created from ctx
with <SSL_new(3)|SSL_new(3)>. Already created SSL objects are not affected.

SSL_CTX_set_tmp_rsa() sets the temporary/ephemeral RSA key to be used to be
rsa. The key is inherited by all SSL objects newly created from ctx
with <SSL_new(3)|SSL_new(3)>. Already created SSL objects are not affected.

SSL_CTX_need_tmp_rsa() returns 1, if a temporary/ephemeral RSA key is needed
for RSA-based strength-limited 'exportable' ciphersuites because a RSA key
with a keysize larger than 512 bits is installed.

SSL_set_tmp_rsa_callback() sets the callback only for ssl.

SSL_set_tmp_rsa() sets the key only for ssl.

SSL_need_tmp_rsa() returns 1, if a temporary/ephemeral RSA key is needed,
for RSA-based strength-limited 'exportable' ciphersuites because a RSA key
with a keysize larger than 512 bits is installed.

These functions apply to SSL/TLS servers only.

NOTES

When using a cipher with RSA authentication, an ephemeral RSA key exchange
can take place. In this case the session data are negotiated using the
ephemeral/temporary RSA key and the RSA key supplied and certified
by the certificate chain is only used for signing.

Under previous export restrictions, ciphers with RSA keys shorter (512 bits)
than the usual key length of 1024 bits were created. To use these ciphers
with RSA keys of usual length, an ephemeral key exchange must be performed,
as the normal (certified) key cannot be directly used.

Using ephemeral RSA key exchange yields forward secrecy, as the connection
can only be decrypted, when the RSA key is known. By generating a temporary
RSA key inside the server application that is lost when the application
is left, it becomes impossible for an attacker to decrypt past sessions,
even if he gets hold of the normal (certified) RSA key, as this key was
used for signing only. The downside is that creating a RSA key is
computationally expensive.

Additionally, the use of ephemeral RSA key exchange is only allowed in
the TLS standard, when the RSA key can be used for signing only, that is
for export ciphers. Using ephemeral RSA key exchange for other purposes
violates the standard and can break interoperability with clients.
It is therefore strongly recommended to not use ephemeral RSA key
exchange and use DHE (Ephemeral Diffie-Hellman) key exchange instead
in order to achieve forward secrecy (see
SSL_CTX_set_tmp_dh_callback(3)).

An application may either directly specify the key or can supply the key via a
callback function. The callback approach has the advantage, that the callback
may generate the key only in case it is actually needed. As the generation of a
RSA key is however costly, it will lead to a significant delay in the handshake
procedure. Another advantage of the callback function is that it can supply
keys of different size while the explicit setting of the key is only useful for
key size of 512 bits to satisfy the export restricted ciphers and does give
away key length if a longer key would be allowed.

The tmp_rsa_callback is called with the keylength needed and
the is_export information. The is_export flag is set, when the
ephemeral RSA key exchange is performed with an export cipher.

EXAMPLES

Generate temporary RSA keys to prepare ephemeral RSA key exchange. As the
generation of a RSA key costs a lot of computer time, they saved for later
reuse. For demonstration purposes, two keys for 512 bits and 1024 bits
respectively are generated.